Ichnos is a general purpose particle tracking code. As such, Ichnos can trace virtual particles in velocity fields. The development of the code is heavily influenced by the projects that Ichnos was developed for, which is primarily particle tracking in groundwater flow fields. In particular, in groundwater flow fields calculated from finite element groundwater models. Yet, groundwater velocity fields from finite difference and finite volume codes can also be used. A key consideration during the development of the code is to handle simulations with multi-million velocity nodes. Therefore the code can be used with velocitiy fields that can be split across many processors.